Owing in part to their relatively high energy densities, light weight, and potential for long lifetimes, lithium-ion (Li-ion) batteries are used extensively in consumer electronics. In fact, Li-ion batteries have essentially replaced nickel-cadmium and nickel-metal-hydride batteries in many applications. Despite their increasing commercial prevalence, further development of Li-ion batteries is needed, particularly for potential applications in low- or zero-emission hybrid-electrical or fully-electrical vehicles, energy-efficient cargo ships and locomotives, aerospace, and power grids. Such high-power applications will require electrodes with higher specific capacities than those used in currently-existing Li-ion batteries.
Currently, carbon-based materials (e.g., graphite) are employed as the predominant anode material in Li-ion batteries. Carbon (C), in the form of graphite, has a maximum or theoretical specific capacity of about 372 milli-Ampere hours per gram (mAh/g), but suffers from significant capacity losses during cycling. Notably, during the first charge cycle, graphite experiences a high level of irreversibility, meaning that a significant amount of lithium ions intercalate into the graphite anode, but do not deintercalate out of the anode upon discharge of the battery.
Silicon-based materials have received great attention as anode candidates because they exhibit specific capacities that are an order of magnitude greater than that of conventional graphite. For example, silicon (Si) has the highest theoretical specific capacity of all the metals, topping out at about 4200 mAh/g. Unfortunately, silicon suffers from its own significant setbacks.
The primary shortcoming of Si-based anode materials is the volume expansion and contraction that occurs as a result of lithium ion intercalation and deintercalation, respectively, during charge cycling of the battery. In some cases, a silicon-based anode can exhibit an increase, and subsequent decrease, in volume of up to about 400%. These high levels of strain experienced by the anode material can cause irreversible mechanical damage to the anode. Ultimately, this can lead to a loss of contact between the anode and an underlying current collector. Another shortcoming associated with Si-based anode materials is their low electrical conductivity relative to carbon-based anode materials.
The use of silicon-carbon composites to circumvent the limitations of pure Si-based materials has been investigated. Such composites, which have been prepared by pyrolysis, mechanical mixing and milling, or some combination thereof, generally include Si particles embedded in or on a dense carbon matrix. The large volume changes in the Si particles upon lithium intercalation, however, can be accommodated by carbon only to a limited degree, thus offering only limited stability and capacity enhancements relative to pure Si-based anodes.
Thus, despite the advancements made in anode materials, Li-ion batteries remain somewhat limited in their applications. Accordingly, there remains a need for improved anodes for use in Li-ion batteries. These improved anodes, and, ultimately, the improved Li-ion batteries, could open up new applications, such as the so-called high-power applications contemplated above. It is to the provision of such devices that the various embodiments of the present inventions are directed.